1. Field of the Invention
The present invention relates to a projection lens unit used for projecting a high definition image in high definition televisions, computer displays and other devices.
2. Description of Related Art
In recent years, a manner in which a small image source is projected and enlarged by a projection lens unit is the mainstream of the measures for obtaining a large screen image of 40 inches or more. Projectors of this projection type are expected to be used in various kinds of fields including personal use and professional use. In particular, high definition images of 100-inch class are desired, for example, in presentation systems and monitoring systems, but requires projection lens units of a high performance.
There is a projector of a front projection type, which projects an image to a front side of a screen so that the screen image is watched in the same style as the moving picture. This projector of the front projection type requires several kinds of adjustment every time the location of the projector and/or the screen size are changed. Therefore, there has been demanded a projection lens unit which can comply with various screen sizes with the single projection lens unit and can be adjusted simply.
The projection lens unit used in the projector must have an F-number close to 1 in order to ensure a sufficient brightness of the projected image. Further, aberration correction must be sufficiently effected through the whole image. When the projection lens unit is formed of only glass lenses in order to obtain a sufficient lens performance in the prior art, such a disadvantage is caused that it is heavy and expensive because many lenses are used. In the prior art using an aspherical plastic lens, since an aspherical amount is large, it cannot be worked easily in the manufacturing process and a sufficient accuracy may not be obtained, so that an intended performance cannot be sufficiently obtained. By these reasons, a light and inexpensive projection lens unit has been desired.
In the prior art, the projection lens unit used in the projector is designed to comply with arbitrary screen sizes, and various kinds of aberration are corrected so as to obtain the optimum optical performance at the designed paraxial magnification. Therefore, when the magnification is varied for changing the screen size, the lenses in the projection lens unit must be partially or entirely redesigned to correct the aberration.
A projection lens unit has been reported that allows projection to various screen sizes only with a mechanism for moving lens elements without re-designing the lenses. This structure, however, can be used only for the zoom ratio of about 2 or less, because the performance of the projection lens unit allowing practically use cannot be ensured when the zoom ratio is larger than 2.
Moreover, a high imaging performance and thus highly accurate aberration correction are required in the projection lens unit used for projecting a high definition image in the high definition television, computer display or others. Therefore, such a design is required that enables variable magnification without changing an optical arrangement of lens elements in order to ensure a sufficient imaging performance while complying with a plurality of screen sizes.
When the projection lens unit is formed of only the glass lenses as described above, many lenses are required, resulting in disadvantages relating to the weight, cost, mounting structure and others. In the design using the aspherical plastic lenses, the aspherical amount becomes large, making the working difficult, and the optical characteristics and the shape change in accordance with the change of ambient conditions such as a temperature and a humidity, resulting in disadvantages, for example, that the designed performance cannot be fully ensured.